Utilizing the magnetoelastic effect of a ferromagnetic material for electrically measuring mechanical forces is an idea that arose more than 50 years ago. However, it was outcompeted by strain gauges, except in special applications in heavy industrial environments where the high signal power and the good mechanical properties of the transducer have made the magnetoelastic transducer practically universally prevailing.
These properties, in combination with the possibility of mass-producing simple and inexpensive but still sufficiently accurate magnetoelastic transducers, has opened up a new niche for the magnetoelastic technique.
U.S. Pat. No. 1,906,551 entitled "Magnetic testing method and means" describes several basic principles of force measurement utilizing the magnetoelastic effect. It describes, inter alia, the use of separate excitation and measuring windings as well as the use of an unloaded measuring body applied to a differential measurement.
The transducer described in U.S. 1,906,551 is a relatively complicated design which is not conceivable for use in simple and robust applications. In addition, the design as such has a number of fundamental deficiencies. For a differential measurement to provide good stability with the aid of an unloaded measuring body, it is required that the measuring body should be as equal to the force-measuring body as possible, both as regards the measuring bodies per se, i.e. material and geometry, and as regards the surrounding magnetic circuits. To obtain good conformity with the temperature, an additional requirement is that there should be very good thermal contact between the two measuring bodies.
U.S. Pat. No. 1,906,551 also describes a pressure measuring device comprising a transducer for tension and/or compression. The purpose of this is to generate a tensile stress in one measuring zone in order to obtain a tension signal with inverse sensitivity to the signal from the other measuring zone which is loaded with compressive stress. If instead of subtracting the signal from an unloaded measuring body, this tension signal is subtracted, theoretically an improved sensitivity and improved linearity are obtained.
With the above-described concept, it has proved that those parts of the transducer for tension and/or compression, which are subjected to compression and bending moments, must be made extremely strong in order to provide, in reality, any significant tensile stress in the measuring zone. This makes the transducer both clumsy and expensive. In addition, the heavy stresses, because of bending in the transducer for tension and/ or compression, result in a very unevenly distributed stress around the magnetic circuit, which in turn leads to a deteriorated linearity of the measured signal.
U.S. Pat. No. 2,867,118 entitled "Transducer and stress measuring means" describes a magnetoelastic transducer in which two measuring zones made of different materials are loaded to the same stress. One of the materials has positive magnetostriction and the other has negative magnetostriction and, as in other transducers, the difference between the magnetic fluxes in the respective measuring zone is measured.
This solution satisfies the requirement that the transducer shall be simple and inexpensive to manufacture. Since the zones are made of different materials, however, the requirement for good conformity between these cannot be fulfilled, which is a necessity in order to obtain good stability according to the above reasoning.
U.S. Pat. No. 4,802,368 describes a transducer for tension and/or compression comprising two identical transducer halves which are screwed together in the middle. In addition to what is mentioned in general terms above concerning transducers for tension and/or compression, the force transducer according to the last-mentioned US patent suffers from the additional disadvantage that both the magnetic field lines and the field of force lines are forced to pass over joints. Since all surfaces are uneven, seen microscopically, the magnetic field will experience different degrees of difficulty in passing the joint at different points of the surface, whereby the good rotational symmetry of the transducer with respect to the magnetic field configuration will be broken. This reduces the ability of the transducer to suppress lateral loads. In addition, the mechanical stresses arising in the transducer upon loading will be unevenly distributed over the joint, and sliding between the surfaces gives rise to hysteresis of the transducer signal.